The present invention is directed to a method for operating an internal combustion engine.
During a cold start of an internal combustion engine, the temperatures of the walls of the intake port and the combustion chamber are markedly lower than the temperature that prevails during normal operation. A portion of the injected fuel condenses on the cold combustion-chamber walls and, initially, does not take part in combustion. Under these conditions, a significant quantity of the injected fuel is scraped into the oil by the piston rings, and a further quantity enters the exhaust-gas system, unburned. As the internal combustion engine and engine oil continue to heat up, the portion of fuel scraped into the oil evaporates into the oil, however, and is directed via crankcase ventilation into the intake manifold and enriches the air-fuel mixture.
To nevertheless ensure a good start, post-start phase and warm-up, a markedly greater quantity of fuel must be injected than is typical when the engine is warm. This excess fuel portion nearly corresponds to the quantity of fuel that is lost, unburned, in the exhaust gas and/or that enters the oil via the piston rings. In addition, the quantity of fuel added is a function not only of the temperature of the internal combustion engine, but also of engine speed and the torque required by the driver. The quantity of fuel added to the oil is therefore greatly increased, e.g., by a forced driving style. The quantity of fuel added also depends on the fuel type. For example, when alcohol is used instead of gasoline, it is observed that a markedly greater quantity of fuel is added that, even when the start temperatures are much higher than zero degrees Celsius, cannot be disregarded. In principle, the quantity of fuel added can be determined based on the evaporation behavior of the fuel. The poorer the fuel evaporation is at engine start-up temperatures, the greater the quantity of fuel is that condenses or remains fluid, and the greater the quantity of fuel is that must be injected.
To compensate for fuel condensation, with gasoline engines, an intervention in the mixture pilot control is carried out, for example, and a greater quantity of fuel is precontrolled, based on enrichment factors. As soon as the lambda closed-loop control is active, it can also adjust this quantity of fuel.
Although more fuel must be injected in the condensation phase when the engine is cold, as described above, the effect is reversed as the oil becomes increasingly hotter. The fuel contained in the oil then evaporates and is supplied to the combustion via crankcase ventilation. The injected-fuel quantity must now be reduced.
If the evaporation rate is low, it is sufficient for the lambda closed-loop control to compensate for this extra flow of fuel mass coming from evaporation that therefore supplements the injected-fuel quantity. It must be ensured, however, that, if there are strong deviations in the lamba closed-loop control, this is not interpreted to mean that a diagnostic fault exists. In particular, it has been demonstrated that, at idle and at operating points close to idle, the evaporation is much more pronounced than at high loads and engine speeds.
Publication DE 44 23 241 A1 makes known a learning closed-loop control method for adjusting the composition of the operating mixture for an internal combustion engine, with which the speed at which the additional interventions is learned is a function of temperature. By way of this method, the situation is prevented, among others, that the portion of gasoline evaporating out of the engine oil during the warm-up phase erroneously influences the mixture regulation. If the oil temperature has been above a threshold for long enough, it is assumed that the gasoline has evaporated, and the closed-loop control method returns to operation based on normal values again.
Furthermore, with injection systems that tolerate gasoline as well as alcohol, and a mixture of the two in any combination, and that adapt the mixture in the tank without an additional sensor—known as “fully adaptive flexible fuel systems”—the mixture adaptation is quasi maintained, when fuel evaporation takes place as expected, and the control stroke of the lambda closed-loop control system is expanded markedly in the downward direction.